1. Field of the Invention
The present invention relates to oxygen concentration detectors (oxygen sensor, air-fuel ratio sensor, leak sensor and such like sensors) effective in use for the control of combustion in an internal-combustion engine.
2. Description of the Related Art
As gas detectors for detecting the oxygen concentration in exhaust gas of an automobile internal-combustion engine, a gas detector using a ZrO.sub.2 solid electrolyte, say, of oxygen concentration electromotive force type has been known since former days and such has been widely made practicable. As the above electromotive force type of gas detector, there is such an oxygen concentration detector as disclosed in Japanese Examined Patent Publication No. 2-15017.
The relevant oxygen concentration detector has an oxygen sensing element provided on the tip thereof, which sensing element is a bottomed cylinder comprising an inside electrode, solid electrolyte sintered body, outside electrode, and electrode protecting layer formed in sequence, whereas a heater is inserted in the internal cavity of the above oxygen sensing element. The above electrode protecting layer is formed of ceramic coating layer or, say Al.sub.2 O.sub.3 layer provided on a ceramic coating layer. Exhaust gas passes through the above ceramic coating layer or the above Al.sub.2 O.sub.3 layer and reaches to the above outside electrode to provide a sensor output. Under certain practical conditions, however, the outermost surface of the above oxygen sensing element is coated with a deposit originating from exhaust gas. This deposit comprises fine particles or glassy films composed of oil components of P, Ca, Zn, Si or the like, and gasoline mixture components of K, Na, Pb or the like, while the surface of the above electrode protecting layer is coated with the above deposit, thereby preventing exhaust gas from scattering to the above outside electrode. This causes a deterioration of quality, such as a decrease in output or a decrease in response. Accordingly, there is a problem in that sticking of such a deposit causes said gas detector to fail in stable sensor characteristic for a long time.
Thus, to solve the problems mentioned above, improving proposition, for example, of having a poisonous substance trap layer comprising a relatively porous flame fusion film or Al.sub.2 O.sub.3 particles of several .mu.m grain size provided on the above electrode protecting layer, thereby eliminating the clogging.
First, an oxygen sensing element has been proposed in which the surface exposed to exhaust gas is coated with an insulating coat comprising a metal oxide, heat-resisting, porous, and permeable for the gas to be detected, said insulating film made to bear a catalyst (Japanese Patent Application Laid-Open No. 52-73089). As the above insulating film, .gamma.-Al.sub.2 O.sub.3, ZrO.sub.2, MgO or the like is employed, whereas Pt, Pd, Rh or the like is employed as the above catalyst. The above insulating coat prevents a poisonous component from sticking directly to the above oxygen sensing element and therefore the gas detector itself improves in durability.
Secondly, an oxygen sensor has been proposed in which on the surface of the electrode for the gas to be detected in an oxygen sensing element is provided a protective layer mixed with .gamma.-alumina particles for covering the catalyst layer (Japanese Patent Application Laid-Open No. 61-153561). According to this constitution, the Pb component contained in exhaust air is absorbed to a highly absorptive .gamma.-Al.sub.2 O.sub.3 particles if permeating the protective layer, so that the poisoning by Pb on the catalyst layer can be prevented.
Thirdly has been proposed a method for forming a relatively porous trap layer for poisoned materials on the surface of an oxygen sensing element by flame fusion coating process (Japanese Patent Application Laid-Open No. 53-13980); fourthly, a method for forming a trap layer containing .gamma.-Al.sub.2 O.sub.3 several .mu.m in grain size for poisons on the surface of an oxygen sensing element (Japanese Patent Application Laid-Open No. 621-153561); fifthly, a method for forming a trap layer comprising a Pt-bearing catalytic layer for poisoned materials on the surface of an oxygen sensing element (Japanese Examined Patent Publication No. 5-76575); and sixthly, a method for forming a Pb-made trap layer having a fixed pore volume and thickness for poisons on the surface of an oxygen sensing element (Japanese Patent Application Laid-Open No. 62-187245).
These gas detectors of the above propositions as described above are effective especially for the detection of exhaust gas in an internal-combustion engine, that is, has a great effect on the capture of poisonous components, such as Pb P S Si and Zn Also, they are effective in preventing the poisonous deterioration of interior catalysts and in greatly promoting the durability.
In recent years, however, further improvement in fuel cost and performance for internal-combustion engines has been forwarded corresponding to the global environment policy and so the using surroundings of a gas detector become severer. The using temperature of gas detectors becomes higher and the amount of poisonous components increases. Consequently, under high temperatures, the trapped poisonous components react with each other, or fuse and form an intrinsically air-tight glassy deposition coat on the surface of oxygen sensing elements on following cooling, causing a clogging.
Based upon a intensive study for coping with these circumstances, it is one object of the present invention to provide an oxygen concentration detector free from clogging and excellent in durability, that is, capable of maintaining a stable sensor output for a long period, by preventing the formation of a dense air-tight glassy deposit coating on the surface of oxygen sensing elements.